Radar systems are used in motor vehicles, for example, for adaptive cruise control, and have the function of locating other vehicles, so that the distance of one's own vehicle from a vehicle traveling ahead is automatically controlled.
A commonly used radar system for these purposes is an FMCW radar (frequency modulated continuous wave). What is involved in this system is a continuous wave radar in which the frequency of the radar signal sent by the antenna is modulated in a ramp-shaped manner. The radar echo, received by the antenna at a certain point in time, is mixed with the radar signal sent at the same point in time, so that by beats one obtains a lower frequency mixer signal whose frequency corresponds to the frequency difference between the sent and the received signal. This frequency difference is a function, on the one hand, of the signal propagation time, and, with that, of the distance of the radar target, and, on the other hand, because of the Doppler effect, of the relative speed of the radar target.
The mixer signal is digitized and is sampled in each measuring cycle over a time span that is approximately equivalent to the duration of the frequency ramp, and is transformed into a spectrum, for instance, by rapid Fourier transformation (FFT). In this spectrum, then, each located radar target is represented by a peak at a certain frequency. The ambiguity between the distance-dependent and the relative speed-dependent frequency proportion is removed in that the sent radar signal in each measuring cycle is modulated using different ramp slopes, for instance, once using a rising ramp and once using a falling ramp having the same slope, as an absolute value. If one then forms the sum of the frequencies of the mixer signal that were obtained for the rising and the falling ramp, the propagation time-dependent proportions mutually cancel out, so that one obtains a measure for the Doppler shift, and therewith the relative speed. If the two frequencies are subtracted from each other, the speed-dependent proportions cancel out, and one obtains a measure for the distance apart.
In order to be able to assign the located vehicles correctly to the various lanes of the roadway, one may work with radar system having angular resolution. For instance, the antenna has several antenna patches situated in the focal plane of a common microwave lens, so that several slightly angularly offset radar lobes are generated. The evaluation described above is then separately carried out for each channel, that is, for each patch, and, by comparison of the amplitudes and/or the phases of the signals included in the various channels for the same radar target, one may at least approximately determine the azimuth angle of the radar target.
In an architecture that has been common up to now, of the evaluation electronics system, one works with only one single signal path, in which the mixer signals of the various channels are digitized and supplied to the first processor, for example, a digital signal processor (DSP), whose main task it is to calculate the corresponding spectrum for each channel. The additional evaluation then takes place using a separate second processor, such as a microcontroller (μC). This microcontroller fulfills manifold tasks, among other things, the removal of the above-mentioned ambiguity, the determination of the azimuth angles of the radar targets over several measuring cycles (tracking), the selection of a target object for the clearance control and finally, the clearance control by intervention in the drive system and/or the braking system of the vehicle.
In order that the traffic situation may be evaluated sufficiently reliably, the radar measurements have to be repeated at a short cycle time, for instance, at a period of a few milliseconds. It follows that the digital signal processing in the processors has to take place at high speed and using a correspondingly high data throughput, so that the appearance of errors in the signal path cannot always be avoided. Such errors may be caused, for example, by interspersed noise signals, bit A1 liasing and the like.
Up to the present, ACC systems have generally been installed for travel at relatively high driving speeds on superhighways or expressways, that is, in situations in which the vehicle clearances are relatively great, the dynamics of the traffic events are comparatively low, and, in particular, one does not have to anticipate standing obstacles in the traffic lane. There have been attempts made, however, to broaden the functionality of radar-assisted driver assistance systems, and especially to extend them into the lower speed range. An example for such a broadening of the function is, for instance, a stop-and-go function which, for example, when driving up to the end of a traffic jam, makes it possible automatically to brake one's own vehicle to a standstill, and which possibly also makes possible automatically driving off again as the congestion lifts. Other examples of function broadening are so-called pre-crash functions, such as the automatic initiation of emergency braking when the danger of a collision is detected, preparatory activation of passive safety systems, such as air bags and belt tensioners, and the like. For these functions, naturally, high safety standards apply, so that even sporadic malfunctions of the radar system are no longer able to be tolerated to the same extent as took place up to now.